I don't know if this will help, but someone on Thriftyfun recommended Dawn dish washing liquid for stains and I just used it to remove human urine stains from the rugs around my commode.

I just took a white old rag and put warm water on it and a few drops of Dawn. I rubbed the stain pretty well, but not for a long time.

I washed the rug with laundry detergent and bleach and for the first time, the urine came out. For the past ten years I just replaced the rugs because I could never get the urine out, no matter what I used.

I never put these rugs with rubber backing in the dryer, I just air dry them.

Depending on the size of the cushion perhaps you could wash it in your washing machine or a large washing machine available at the laundromat on the gentle, cold water cycle and make sure it is dried completely afterwards. If you're worried about shrinkage then allow to air dry.

Major pet stores sell a product called "odor ban." It's absolutely wonderful. It's made specifically for urine, or any other organic matter. Bring your cushion outside and just pour it on...making sure it's thoroughly soaked through to the inside. No need to rinse it....just let it dry a couple of days. Odor Bans' purpose in life is to "eat" anything organic. This same concept can be also used on bed mattresses, furniture etc. It worked and I was very, very impressed.